This invention relates generally to solid state lighting applications and specifically to nanocrystalline based phosphors suitable for excitation by Light Emitting Diodes (LED""s) and photonic lighting devices based thereon.
Because of their energy efficiency, LED""s have recently been proposed for lighting applications, particularly for specialty lighting applications, where energy inefficient incandescent and halogen lights are the norm. To date, three main approaches have been taken to provide so called xe2x80x9cwhitexe2x80x9d light from LED""s. The first approach uses clusters of red, green and blue (RGB) LED""s, with color mixing secondary-optics, to produce white light. This approach does provide good quality white light with a xe2x80x9ccolor rendering indexxe2x80x9d (CRI) of xcx9c85 and is energy efficient, however, the need to drive three separate sets of LED""s requires complex and more expensive driver circuitry. The complexity arises due to considerably different extent of degradation in efficiency with increasing temperature, for each of the red, green and blue LEDs and to different degradation lifetimes between the red, green and blue LEDs. Furthermore, high-brightness (5 mW to 1000 mW LED lamp) blue and green LED""s have only recently been developed and are expensive when compared to red LED""s.
A second approach to the generation of white light by LED""s is the use of a high-brightness blue LED (450 nm to 470 nm) to energize a yellow phosphor, such as Yttrium aluminum garnet doped with cerium (YAIG:Ce called xe2x80x9cYAGxe2x80x9d). While this approach is energy efficient, low cost and manufacturable, it provides a lower quality white light with color temperature (CT) of xcx9c7000 K and CRI of xcx9c70 to 75, which is not acceptable for many high quality applications. The use of a thicker phosphor layer to absorb and down-convert more of the blue emission, can lower the color temperature and thereby improve the quality of white light. However, this results in a lower energy efficiency. Alternately, using a single or multiple phosphors with red emission in addition to yellowish-green (or greenish-yellow) emission can increase the color rendering index and thereby improve the quality of white light yielding a CT of xcx9c4000 K and CRI of xcx9c80 to 85 but with lower energy efficiency. However, optical efficiency of the phosphor containing package is only about 50%, resulting in decreased light extraction in each of the above cases.
A third approach to the generation of white light by LED""s is the use of a high-brightness UV/violet LED (emitting 370-430 nm radiation) to energize RGB phosphors. This approach provides high quality white light with CRI of xcx9c90 or higher, is low cost and is reliable to the extent that the encapsulant in the package, containing/surrounding the phosphor and LED chip/die does not degrade in the presence of UV/violet emission. This is due to shorter degradation lifetimes and a larger decrease in efficiency with increasing ambient temperature, for red LED chips compared to UV/violet or blue LED chips, which leads to greater color-maintenance problems and requires more complex driver circuitry. However, at present this approach has very poor efficiency because of the poor light conversion efficiency of the UV/violet excitable RGB phosphors currently in use. In addition, the optical efficiency of the phosphor containing package is only about 50%, resulting in a further decrease in light extraction.
The present invention is directed to providing nano sized RGB (and any other visible wavelength emitting.) UV/violet excitable phosphors or RG (and any other visible wavelength emitting) Blue excitable phosphors of a higher light down conversion efficiency, which in turn provides a higher energy efficiency from a lighting system based on UV/blue LED""s (370 nm to 470 nm).
In conventional activator-based phosphors, the absorption of the exciting radiation is primarily provided by the host while the activator generally dictates the characteristic emission. Because the excited electrons are transferred from host to the activator, the emission characteristics (e.g. emission wavelength) are not so critically dependent on the host, yet the absorption, oscillator strength, efficiency etc. are critically dependent on the host.
Currently used phosphors were developed and optimized over the past 30 years for two major applications. (1) Fluorescent lamps that utilize 254 nm UV radiation from Hg discharge and (2) For CRTs where the RGB-phosphors were excited with an electron beam. In both the cases the emphasis was on the phosphor efficiency of the characteristic radiation, which depends critically on the efficient transfer of the host electron to the activator. One major requirement on the activator was that it must be a localized impurity so as to yield narrow emission (line-emitter). The fact that the localized impurities (e.g. rare-earth metals) are extensively used for this purpose, overall luminescent efficiency (transition rate) remain critically dependent on the host crystalline structure (symmetry and selection rules) and the electronic structure (crystal field splitting) of the localized atom.
The present invention is directed to a photonic structure for xe2x80x9cwhitexe2x80x9d light generation by phosphors under the excitation of a LED. The photonic structure mounts the LED and an optically transparent matrix having dispersed therein phosphors which will emit light under the excitation of the radiation of the LED. The transparent matrix may include nanoparticles for matching the index of refraction of the material of the matrix to that of the light generating phosphors. The matrix material may be readily formed by molding and formed into a variety of shapes including lenses for focusing the emitted light. A large number of the photonic structures may be arranged on a substrate to provide even illumination or other purposes. The phosphors dispersed in the matrix are preferably nanocrystalline.
The present invention provides:
A High refractive index ( greater than 1.8, xcx9c1.9), optically clear, transfer moldable encapsulating lens
Dimensions larger than xcx9c1 cm are achievable
Encapsulation of InGaN (Blue/violet/green)and AlInGaP (red/yellow) saturated color LED dies.
Encapsulation of white LED lamps containing an InGaN LED die and conventional phosphors
A QCA nanophosphor photoluminescent downconverter:
QCA nanophosphors with brightness equal to, or greater than, conventional phosphors
Line and Broadband emitting QCA nanophosphors
Optically transparent and transfer moldable photoluminescent downconverters excitable in the 370 nm to 470 nm wavelength regime
Self-assembled structures with sub-wavelength dimensions, formed using QCA nanophosphors, as an enabler for photonic bandgap structures
A downconverter for high-power, high-flux LED lamps, capable of operating with excitation LED die/chip junction temperature in excess of 80C and excitation intensity well in excess of 1020 tons per cm2 per sec.